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1 How the properties of galaxies are affected by the environment? Reinaldo R. de Carvalho - DAS/INPE Marina Trevisan Reinaldo Rosa The activities in this project follow from the Tatiana Moura general context of the main subject that is Diego Stalder the evolution of ETGs in different André Ribeiro Francesco La Barbera Roy Gal Gary Mamon Joe Silk environments up to z 0.5, which corresponds to an epoch of 5 billion years ago.

5 Galaxy Formation - A Brief History Monolithic collapse and merge I - In the 1930 s we already knew that there were two types of galaxies, E and S. 2 - Eggen, Lynden-Bell & Sandage (1962) - galaxies form from the collapse of gas clouds, and the difference between E and S reflects the rapidity of star formation during the collapse. τsf / τco >> 1 SPIRALS τsf / τco << 1 ELLIPTICALS

6 Galaxy Formation - A Brief History

7 Toomre & Toomre (1972) Numerical simulations demonstrate convincingly that some of the extraordinary structures seen in peculiar galaxies, such as long tails, could be produced by tidal interactions between two normal spirals. Based on the observed frequency of galaxies with such signatures of interactions, and on their estimate of the time scale over which tidal tails might be visible Most elliptical galaxies could be merger remnants. In an extreme version of this picture, all galaxies initially form as disks, while all ellipticals are produced by mergers between pre-existing galaxies

8 GF in Dark Matter Halos By the end of the 1970s, several lines of argument had led to the conclusion that dark matter must play an important role in galaxy formation. White & Rees (1978) proposed a two-stage theory for galaxy formation: 1) dark halos form first through hierarchical clustering; 2) the luminous content of galaxies results from cooling and condensation of gas within the potential wells provided by these dark halos.

9 GF in Dark Matter Halos The model of White and Rees contains the basic ideas of the modern theory of galaxy formation. Feedback is required to explain the low overall efficiency of galaxy formation - supernova feedback operates in dwarf galaxies. Efstathiou & Silk (1983) discussed how the two-stage theory of White & Rees (1978) can solve some of the problems in earlier models based on the collapse of gas clouds. They argued that extended dark matter halos around galaxies make mergers of galaxies more likely, a precondition for Toomre & Toomre s (1972) merger scenario of elliptical galaxy formation to be viable.

10 Environment - How to characterize? Environment has been characterized over the years in different ways and it seems like a simple thing to define. Local density Distance from the center Is the velocity distribution gaussian? Halo Mass

11 ion o. o i- in ot, as he t d rt f has a local minimum at m, which implies that f is bimodal. Conversely, if I12 - li < 2c, then both fl and f2 are concave down at m, hence so is f. Thus, f has a local maximum at m and is therefore unimodal. In the borderline case 1t2 - second derivative of f vanishes, but the fourth derivative can be used to show that f has a maximum at m and thus is unimodal. Environment - 11 How = 2a the to characterize? How Difficult or Easy is to distinguish Gaussians? a m Figure 4. (a) When pi1 - I 21<2a, b m both normal components (light lines) are concave down at m = (i 1 + IL2)/2, so the mixture f (heavy line) is also concave down at m and is therefore unimodal. (b) When Il1-12 1>2a, both components are concave up at m, so the mixture f is also concave up at m, hence is bimodal. The dots indicate inflection points of the component densities. I - -Imb.- - am, -, - Figure 4 illustrates this argument that a two standard deviation separation between the means is needed for bimodality. The symmetry in Figure 4 is crucial and so this proof does not II

Chapter 15.3 Galaxy Evolution Elliptical Galaxies Spiral Galaxies Irregular Galaxies Are there any connections between the three types of galaxies? How do galaxies form? How do galaxies evolve? P.S. You

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